The present invention relates to a bandgap reference circuit and a power supply circuit, and more specifically, to a bandgap reference circuit and a power supply circuit that correct temperature characteristics.
In recent years, hybrid cars and electric vehicles have become popular, and more and more vehicles are loaded with batteries in order to obtain electric power. Such a vehicle typically uses an assembled battery including a large number of battery cells connected in series in order to obtain high voltage. The voltages of the battery cells of the assembled battery fluctuate according to use conditions of the vehicle, as is similar to gasoline in gasoline cars. Accordingly, a system for monitoring voltages is necessary to monitor the status of the battery cells.
A voltage to be monitored is input to a voltage monitoring system as an analog signal. The voltage monitoring system performs analog to digital conversion (hereinafter referred to as AD conversion) to convert the analog signal to a digital signal. Therefore, an analog to digital converter (hereinafter referred to as ADC) is included in the voltage monitoring system and an apparatus or a circuit in the voltage monitoring system.
For the safe travelling of hybrid cars or electric vehicles, it is required to monitor the output voltage of the assembled battery with high accuracy. Therefore, an increase in the accuracy of the AD conversion by the ADC is required. In order to increase the accuracy of the AD conversion by the ADC, it is required to suppress fluctuations in the reference voltage supplied to the ADC. Accordingly, a bandgap reference circuit (hereinafter referred to as BGR) with little voltage fluctuation is used as a reference voltage source.
Hereinafter, a typical BGR (specification of U.S. Pat. No. 3,887,863) will be described. FIG. 24 is a circuit diagram showing a configuration of a typical BGR circuit 1100. The BGR circuit 1100 is a BGR circuit which is generally called a Brokaw cell. The BGR circuit 1100 includes resistors RL101 and RL102, bipolar transistors Q101 and Q102, resistors R101 and R102, and an amplifier AMP.
The resistor RL101 is connected between a power supply terminal that supplies a power supply voltage VDD (hereinafter referred to as a power supply terminal VDD) and the collector of the bipolar transistor Q101. The resistor R101 is connected between the emitter of the bipolar transistor Q101 and a power supply terminal that supplies a ground voltage GND (hereinafter referred to as a ground terminal GND). The base of the bipolar transistor Q101 is connected to an output terminal TOUT.
The resistor RL102 is connected between the power supply terminal VDD and the collector of the bipolar transistor Q102. The resistor R102 is connected between the emitter of the bipolar transistor Q102 and the emitter of the bipolar transistor Q101. The base of the bipolar transistor Q102 is connected to the output terminal TOUT.
The non-inverting input of the amplifier AMP is connected to the collector of the bipolar transistor Q101, and the inverting input of the amplifier AMP is connected to the collector of the bipolar transistor Q102. The output of the amplifier AMP is connected to the output terminal TOUT.
Note that the bipolar transistor Q101 and the bipolar transistor Q102 have different sizes. In this example, the area ratio of the bipolar transistor Q101 to the bipolar transistor Q102 is 1:N. Accordingly, the bipolar transistor Q101 and the bipolar transistor Q102 have different current densities during operation. In summary, the current density J101 of the bipolar transistor Q101 and the current density J102 of the bipolar transistor Q102 satisfy the relation shown below in formula (1).
                                          J            102                                J            101                          =        N                            (        1        )            
Subsequently, an operation of the BGR circuit 1100 will be described. In the following description, the base-to-emitter voltages of the bipolar transistors Q101 and Q102 are denoted by VBE1 and VBE2, respectively. As shown in FIG. 24, a current I1 flows through the bipolar transistor Q101, and a current I2 flows through the bipolar transistor Q102 and the resistor R102. A current I flows through the resistor R101. In this case, an output voltage VBGR that appears in the output terminal TOUT is expressed as the following formula (2).VBGR=VBE1+R101·I  (2)
The base-to-emitter voltage VBE1 of the bipolar transistor Q101 can be expressed by the following formula (3).VBE1=VBE2+R102·I2  (3)
Solving formula (3) for the current I2 yields the following formula (4).
                              I          ⁢                                          ⁢          2                =                                            V                              BE                ⁢                                                                  ⁢                1                                      -                          V                              BE                ⁢                                                                  ⁢                2                                                          R            ⁢                                                  ⁢            102                                              (        4        )            
Further, (VBE1−VBE2)=ΔVBE is expressed by the following formula (5). Note that K is Boltzmann constant, q is the charge of an electron, and T is absolute temperature.
                              Δ          ⁢                                          ⁢                      V            BE                          =                              KT            q                    ⁢                      ln            ⁡                          (                                                J                  102                                                  J                  101                                            )                                                          (        5        )            
Using formula (1), formula (5) can be rewritten into the following formula (6).
                              Δ          ⁢                                          ⁢                      V            BE                          =                              KT            q                    ⁢                      ln            ⁡                          (              N              )                                                          (        6        )            
Substituting formula (6) into formula (4) yields the following formula (7).
                              I          ⁢                                          ⁢          2                =                              KT                                          q                ·                R                            ⁢                                                          ⁢              102                                ⁢                      ln            ⁡                          (              N              )                                                          (        7        )            
The BGR circuit 1100 operates so that the current I1 becomes equal to the current I2. When I1=I2, the following formula (8) is established.I=2·I2  (8)
From formulae (2), (7), and (8), the following formula (9) can be obtained.
                              V          BGR                =                              V                          BE              ⁢                                                          ⁢              1                                +                                    2              ·                                                R                  ⁢                                                                          ⁢                  101                                                  R                  ⁢                                                                          ⁢                  102                                            ·                              KT                q                                      ⁢                          ln              ⁡                              (                N                )                                                                        (        9        )            
The BGR circuit 1100 is able to correct temperature dependencies of bipolar transistors. Based on formula (9), the temperature dependencies of the bipolar transistors appear as fluctuations in VBE1 due to temperature changes. The second term of the right side of formula (9) is a term which indicates the effect of correcting fluctuations in VBE1. In summary, the second term of the right side of formula (9) having a positive temperature coefficient acts on the base-to-emitter voltage VBE1 of the bipolar transistor Q101 having a negative temperature coefficient, thereby being able to correct the temperature dependencies of the output voltage VBGR.
Various other BGR circuits have been proposed. The specification of U.S. Pat. No. 7,420,359 discloses a method of referring an output voltage of a BGR circuit to supply a signal according to the reference result to the BGR circuit, thereby correcting the output voltage of the BGR circuit. The specification of U.S. Pat. No. 6,642,699 discloses a BGR circuit that compensates temperature characteristics using a differential pair. The specification of U.S. Pat. No. 6,118,264 discloses a method of adding a correction voltage to an output voltage of a BGR circuit, to compensate the output voltage of the BGR circuit.